DEEPER PI DETECTION DEPTH

[Davor]

Hypotheses (not complete):
...
Aziz

11. Engineers are in deep doo-doo if their chief does not understand what the heck they are talking about. (which happens a lot)
12. Only the competition is entitled to radical design changes. (no competition - no changes)

[Davor]

Years ago, I used this pre-amp. It was giving good results. However, because of latent temperature drift etc. I abandoned the design.

Your design is noninverting one and both inputs are exposed to full signal swing and rails limitation. The solution I propose is an inverting one, and opamp via its feedback fights the full wrath of monocoil Pi voltage. OK, there is one diode to help it - just in case. But the whole point is that you can have a full PI decay exposed to analysis and decomposition. It would be funny seeing it on scope.

(I'm still lab-less )

My personal view of PI so far is that there is far too much power in Tx, while the only thing that really matters is di/dt or voltage. Even in that department there is too much juice flowing.

[Davor]

I'd go along with Earl's observations, except that I'm not too much into the noise story. Yet.

Problem is in metal detectors you want to measure difference between 1.0000 and 1.0001 signal ....

...which you can only benefit from the log amp. Log amp detectors are famous for AM detection of low modulation indexes.

I still think the whole story about a nasty noise is far too much exaggerated because I can decide NOT to sample deep in noise but half a decade above. It is the slope that matters, and simulation shows nice splitting of curves waaaay above noise. Besides, even a traditional PI Rx does not go that deep into noise. There is a tendency to sample a bit earlier.

I think the perfect way of harnessing data from such a setup is by means of window comparators. Level reference instead of time, and time result instead of level. Two such windows could yield discrimination. Voltage reference for the windows can be set by diode forward voltage drop, hence a temperature compensation. Easy.

Log amps are a different pack of animals altogether, and they need different care and feeding. But they are not saturating. And they are darn fast.

[Altra]

I had saved this post from Eric Foster. It came from the the PI forum on Findmall.
Maybe helpful comming from someone who has actually tried a logamp?

Quote.
Hi Randy and All,
I would be very interested in the log amp you mentioned. Do you have the part number? This is a good way to go, either for DSP or normal analogue processing. Although linear amps are invariably used, the signal strength varies so dramatically with object range, as a result of the sixth power law, that they rapidly run into saturation. The other benefit of a log amplifier is that it is possible to enhance the differences in the decay curve between ferrous and non-ferrous metals. A non-ferrous object has a decay that is exponential in shape for later times i.e. times longer than one time constant. Run this through a log amp and you get a linear decay. The decay from a ferrous object when run through a log amp is anything but linear. If you take a long sample window of the signal and differentiate it, then non-ferrous objects come out as a constant reading, depending on the slope of the linear decay, which depends on the time constant. Hence an indicator could be calibrated for different coins etc. Ferrous objects could be distinguished by the fact that the reading changes during the sample window as the slope changes. Tomorrow I will try and post some log plots of different objects which were taken in 1968 using a wide band log amplifier (yes it was an IC!) which clearly shows the differences which one would be hard pushed to notice on a linear display.
So why isnt this system currently used? The main bugbear is noise. As the signal decays into the noise, the gain of the log amp goes up and with no signal the output is flapping about all over the place. Cant have a log of zero or negative values. The gain of the amplifier would have to be curtailed or its function otherwise inhibited when no signal is detected. The other problem, which has likely been overcome, is temperature stability. The amp I tried was good as long as the ambient temperature didnt change.
Ive been looking for a good modern log amp for some time to re-evaluate this approach. Perhaps this is it?
Eric.
End Quote

[moodz]

Log amps = Bog amps

Myth Buster
Myth 1 : log amps are expensive ..

You can buy the AD8307 92 db logamp DC to 500 Mhz on ebay for $2 each. They are excellent ... at least they are excellent when used for right application.

Myth 2 : log amps are useful for sensitive metal detectors ..

Log amps are used for things like Radars where your reciever is very sensitive and you want to measure the difference between a very weak signal and very strong signal ( like 10000 times as strong ) Problem is in metal detectors you want to measure difference between 1.0000 and 1.0001 signal .... log amps are wrong amp for this application LOL.

comment .... even in some radio applications where they used to use log amps they now use very high resolution ADC ....since you can have over 100 db of dynamic range ( ie smallest to biggest signal ) ... you dont need a log amp.

moodz

[Tinkerer]

Years ago, I used this pre-amp. It was giving good results. However, because of latent temperature drift etc. I abandoned the design.

The Opamp does not saturate, because of the soft clipping by the transistor feedback. The negative TX pulse that we do not need, is clipped.

The response is somewhat logarithmic. The gain is enhanced for the low voltage here we need it.
The higher voltage has less gain and still higher, the signal is soft clipped so that the opamp does not saturate.

Possibly the latent problems could be solved and then it would make a good alternative.

Tinkerer

[Aziz]

Hi all,

let's apply the Occam's Razor to the log-amps (see more http://en.wikipedia.org/wiki/Occam's_razor ).

"Occam's razor (also written as Ockham's razor, Latin lex parsimoniae) is the law of parsimony, economy or succinctness. It is a principle urging one to select among competing hypotheses that which makes the fewest assumptions and thereby offers the simplest explanation of the effect."
(Quote from Wikipedia)

Question:
Why it (the log-amp) hasn't been used in the top performing detectors yet?

Hypotheses (not complete):
1. Engineers are bloody dumb and do not know that log-amps exist (I bet a lot of them are really dumb).
2. Engineers do know the log-amp but can not migirate the log-amp into the MD application.
3. Engineers do not believe, that it has any potential ("gut feeling").
4. Engineers do not like new ideas ("Never touch a running system!!!"). Why bother with speculative designs, if there is a working solution already?
5. Engineers don't know deep math except the basic calculus (*,/,+,-).
6. Engineers do not really exist. So everybody who is working in the art is a bloody amateur.
7. No one has tried it before.
8. There is a reasonable ground not to use it.
9. There isn't a reasonable ground not to use it.
10. The weather was fine and the girls were very sexy. Engineers didn't have time to look at it.

So, why the f... h... hasn't the log-amps been used yet?

Aziz

[Davor]

I know, but with all other effects in play the temp drift is the least of my worries. Besides, dedicated multi-decade log amplifiers are incredibly expensive, and same as this one must hit the noise floor ... eventually.
Using BJT in a feedback loop is not famous for stability so simple diodes were a logical choice. Noise is also not high on my worries list because it seem as if the most interesting part of the curve is waaaaay above 1mV of a PI decay.
I think the most of the minor problems will be tackled with the tricks already in our toolbox, and we don't need to call them different names. EF can stand for a slowly decaying value past the log slope knee (the ski-like bend) e.g. low signal + offset + noise. That should keep it a bit more stable.
Another possibility may be provided by mere window comparators, and a set of voltage values, hence time differences. That could do as an ADC in a way - a simple counter.

It can work as well with bipolar pulses with only minor changes, and i wonder what it will be like with TEM pulses.

[Aziz]

It seem only appropriate to continue here where we left some time ago. Now this is big. I present you a LTspice simulation of a logarithmic weighted PI Rx with dead time of less than 2 microseconds. Furthermore, its output is logarithmically compressed so there will be much fewer bits required for ADC, where required.

There are some additional repercussions of this approach that enable some very interesting solutions to discrimination (say, you may play with C3 and observe consequences), but finally it is THE weighted solution I was mumbling incoherently for the past few months. Also, it is an inverting solution and thus the signal decays opposite to the "Surf" solution.

On the schematic you'll find two switches that are used to divert signal to a common node, and prevent two Rx-es to interfere with each other, and to draw output in nice colours. The opamp with a bunch of diodes is a log detector: nothing fancy, but works as a horse. I'm aware this is going to perform a bit differently with a real world opamp, but here you go - a place to start experimenting. This can sample very early.

Patent trolls - keep off!!!

Hi Davor,

this kind of log-amp is horrible noisy and unstable (temp drift). Nevertheless, it could still be sufficient for our purpose. Someone should try this log-amp and report the results here.

Cheers,
Aziz

[simonbaker]

It seem only appropriate to continue here where we left some time ago. Now this is big. I present you a LTspice simulation of a logarithmic weighted PI Rx with dead time of less than 2 microseconds. Furthermore, its output is logarithmically compressed so there will be much fewer bits required for ADC, where required.

There are some additional repercussions of this approach that enable some very interesting solutions to discrimination (say, you may play with C3 and observe consequences), but finally it is THE weighted solution I was mumbling incoherently for the past few months. Also, it is an inverting solution and thus the signal decays opposite to the "Surf" solution.

On the schematic you'll find two switches that are used to divert signal to a common node, and prevent two Rx-es to interfere with each other, and to draw output in nice colours. The opamp with a bunch of diodes is a log detector: nothing fancy, but works as a horse. I'm aware this is going to perform a bit differently with a real world opamp, but here you go - a place to start experimenting. This can sample very early.

Patent trolls - keep off!!!

Nice creativity, good contribution!

-SB

[Davor]

It seem only appropriate to continue here where we left some time ago. Now this is big. I present you a LTspice simulation of a logarithmic weighted PI Rx with dead time of less than 2 microseconds. Furthermore, its output is logarithmically compressed so there will be much fewer bits required for ADC, where required.

There are some additional repercussions of this approach that enable some very interesting solutions to discrimination (say, you may play with C3 and observe consequences), but finally it is THE weighted solution I was mumbling incoherently for the past few months. Also, it is an inverting solution and thus the signal decays opposite to the "Surf" solution.

On the schematic you'll find two switches that are used to divert signal to a common node, and prevent two Rx-es to interfere with each other, and to draw output in nice colours. The opamp with a bunch of diodes is a log detector: nothing fancy, but works as a horse. I'm aware this is going to perform a bit differently with a real world opamp, but here you go - a place to start experimenting. This can sample very early.

Patent trolls - keep off!!!

[Davor]

Maybe I was not too straightforward in my explanation, so I suggest exploring a Tayloe mixer as a perfect example what a chopper can do.

A Tayloe mixer uses successive sampling of typically 4 pulses in a row at 90° intervals of a RF signal. 0 and 180° are used for I and 90° 270° are used for Q. It is also possible to use any number of samples, as long as they are used with respective phases.

In PI signal decomposition you don't have phases but time deltas, and your sampling will run a finite number of samples after each PI pulse, thus in effect running cyclically.

Your design already has a balanced input and a ground reference (centre tap), hence your choppers do not need to alternate between signal and ground, so it is a perfect candidate for Tayloe style sampler. Tayloe is usually realised with bifilar coiled transformer with centre tap (double balanced). With Tayloe, samples are stored in capacitors that - in a succession of many samples - act as a LPF. I'm thinking of ~20Hz range. Any garden variety opamp can deal with that. They can also be anti-Tayloed at a pace of your need, say ADC clock, and you can do even more magic with them in DSP. Or just do some plain old boring math and spit out indications for size and a kind of metal.

I think you could as well do with only one sampling window, but why not more?! In case of the above piece of schematic, just replace R7 and R9 with analog switches gated at PI window sample, and a C between them replace with two signal ground referenced capacitors dimensioned for ~20Hz LPF.

PI signal is very orthogonal against the Tx pulse by virtue of non-concurrency. Even with monocoil. It can be fixed to do much more and better than IB VLF. I just know that.

[Davor]

We do NOT WANT: Clipping
Saturation
Overloading of the pre-amp.

The biggest point is that you actually don't have a 1us target response, only a concept that forces you to think into that direction. By my standards you already have far too much amplification in any "classical" PI design. Noise level is at, say uV levels, and signal is near mV levels, hence by applying extra gain you just decrease your dynamic range.

What actually is bothering you much more than everything else is a floating ground, and that is your main enemy with opamp in frontend.

There is a simple solution to many problems, and much more, if you decide to drop the high gain opamp. It is a chopper. Ready-to-wear chopper stabilised opamps are slow and expensive, but it is not a big deal to make your own implementation using analog switches. You don't have to be extremely accurate, so your design may be fast enough to properly sample a PI signal. So instead of one or two windows as for discrimination purposes, you could have, say 10 windows, each supplied to an alternating opamp input, or keep them in SAH-style analog buffer for bucket-brigading into something slow and accurate.
If you are only into high amplification, then just chopper-sample into an alternating signal, amplify, and anti-chopper into a perfectly windowed and perfectly amplified PI Rx windowed signal. Or something in between. Say, two perfectly amplified PI Rx windows for discrimination purposes. You can even apply some weighting function to samples to make them even more interesting.


What is the needed bandwidth anyway?

That would be related to your integration window. Incredibly fast pulses (chopper) can be successfully integrated by incredibly slow integrators. In case you apply some averaging scheme using samples from several PI pulses, then it goes longer and loooonger. You ca'nt make it too long because metal detecting is a somewhat physically active thing, but I'd say that ~20 Hz could become practical. Same as with VLF IB.

[Tinkerer]

What should a perfect pre-amp stage look like?

Hi all,

I do not want to demotivate you but you have to cope with other facts too:
The non-linearity produced by the processing electronics:
- (ADC/DAC)
- (pre-)amplifier
- demodulator
...(and so on)

Consider the following in the amplifier stage only:
Clipping/saturation/overloading of the amplifier is distorting your signal and hence producing harmonic distortion to your signal (spectral blur). Slow amplifiers causing distortion and low-pass filter. Fast amplifiers tend to resonate (oscillator).
Even if you have a perfect ground balance formula, it's worth nothing without compensations to the non-linearities occuring.

Dream on guys. There ain't perfect ground balance. But a better GB (a better approximation solution) is available.

Cheers,
Aziz

Aziz, Davor, Midas, Moodz and many other voices point out the possibility that the preamp stage could be improved a lot. It is time to do it.

We do NOT WANT: Clipping
Saturation
Overloading of the pre-amp.

We want to use an opamp that is fast enough, but not too fast.

We want to use the maximum amplification, while staying within these parameters.

Would using a +/- 12V supply instead of +/-5V, help? More amplification, without clipping and saturation.

Limiting the bandwidth to the actual needed one, would help.

What is the needed bandwidth anyway?

If we want to resolve a 1us target, we should aim for 1MHz. But, can we build a coil with a self resonating frequency of 1MHz? If the coil can only resolve 250kHz, we can also reduce the pre-amp bandwidth to the same frequency.

Reducing the bandwidth, increases the dynamic range.

What else can we do???

Attached is a proposal for a differential input pre-amp, to go with the TEM TX shown above.

Can it be improved? For sure it can, a lot.

Suggestions?????????????

Tinkerer

[Aziz]

Hi all,

I do not want to demotivate you but you have to cope with other facts too:
The non-linearity produced by the processing electronics:
- (ADC/DAC)
- (pre-)amplifier
- demodulator
...(and so on)

Consider the following in the amplifier stage only:
Clipping/saturation/overloading of the amplifier is distorting your signal and hence producing harmonic distortion to your signal (spectral blur). Slow amplifiers causing distortion and low-pass filter. Fast amplifiers tend to resonate (oscillator).
Even if you have a perfect ground balance formula, it's worth nothing without compensations to the non-linearities occuring.

Dream on guys. There ain't perfect ground balance. But a better GB (a better approximation solution) is available.

Cheers,
Aziz